1. Field of the Invention
The present invention relates to a method of detecting structural abnormality of a structure composed of an electrically conductive crystalline or non-crystalline substance having no bandgap, or more specifically to a method of detecting, with higher accuracy, the distribution and directivity of structural abnormality caused by stress in a structure by transmitting piezoelectricity and/or pyroelectricity (electrical energy) directly from the structure, which piezoelectricity and/or pyroelectricity are generated in the structure due to mechanical energy such as pressure and/or heat applied thereto.
The structural abnormality referred to in the present invention includes all of those that are undesirable in a structure, such as strain, step shear (slip), cracking, etc.
2. Description of the Prior Art
One of the conventional methods of detecting structural abnormality (strain in particular) of a substance is to convert the displacement in the substance to an electrical quantity (voltage or current), the displacement being caused by piezoresistance, piezoelectricity, pyroelectricity, electrostatic capacity, magnetism and/or resistance. A strain gauge, piezoelectric pickup, slide rheostat [slide resistor (wire), slip ring], semi-conductor strain gauge, capacitor, differential transformer or Hall element is normally used as a converter for this purpose. The strain gauge is mounted directly on the substance (structure). When pressure or heat is applied to the substance placed in the electrical (or magnetic) field, stress (shear stress, tensile stress, compressive stress) is generated, causing strain of the substance. Resistance fluctuation caused by the strain is detected by the strain gauge. The piezoelectric pickup utilizes the piezoelectrical characteristic of a crystalline substance. It may be mounted directly on the subject substance or at a specified distance from the substance, and converts stress generated in the substance to a quantity of electricity.
According to the above-described conventional methods, however, the strain gauge and the piezoelectric pickup only indicate resistance fluctuation and electrical vibration, respectively, providing no accurate information as to position and degree of the mechanical structural abnormality in the substance. Namely, the observer can obtain structural change-indicating electrical signals from the strain gauge or the piezoelectric pickup mounted on the substance when the mechanical structural abnormality is large enough to be sensed by the strain gauge or the piezoelectric pickup. Since this electric signal has nothing to do with the electrical state of the subject substance, the observer cannot determine the electric polarization (piezoelectricity and pyroelectricity) caused by mechanical structural abnormality in the atomic and molecular structure of the substance. Therefore it is impossible to detect abnormalities such as atomic and molecular structural change in the substance.
Even if electrical circumstance may allow the electric field to be applied to the substance itself as a strain gauge so as to detect the mechanical structural abnormality in the substance in terms of electric resistance fluctuation, the position and degree of the structural abnormality cannot be identified accurately, that is, strain of the atomic and molecular structure in the substance cannot be detected.
As a piezoelectric pickup for the above conventional detecting method, a crystalline substance with elastic compliance such as Rochell salt, barium titanate or ammonium dihydrogenphosphate is usually selected. It is generally believed, however, that neither these non-dielectric crystalline substances (including electrical conductors having any degree of conductivity that is not dielectric and having no bandgap) nor the non-crystalline substances generate piezoelectricity and pyroelectricity. Because of this belief, picking up piezoelectricity or pyroelectricity from crystalline or non-crystalline substances that are non-dielectric electrical conductors has not been practiced in the industry to detect structural abnormality on the basis of observation of the piezoelectricity and/or pyroelectricity.
The object of the present invention is, focusing on the fact that piezoelectricity and/or pyroelectricity are generated in virtually non-dielectric crystalline or non-crystalline substances (including conductors having any degree of electrical conductivity which does not provide a dielectric property and bandgap), to provide a method of accurately detecting structural abnormality in a structure composed of the above-mentioned substance, said abnormality being caused by pressure or heat applied to the structure, and said method comprising the steps of transmitting piezoelectricity or pyroelectricity from the structure and analyzing it.